Polyphenylene ether resins are high performance naturally flame retardant engineering thermoplastics having relatively high melt viscosities and softening points (i.e., in excess of 200.degree. C.). They are useful for many commercial applications requiring high temperature resistance and can be formed into films, fibers and molded articles. However, the art has recognized that polyphenylene ethers have a major drawback which is that they can not be easily melt processed and foam molded, particularly to very low densities.
Polyphenylene ethers have been combined with polystyrene resins to improve moldability. For example, Cizek, U.S. Pat. No. 3,383,435 discloses that improved processability of polyphenylene ether can be obtained by the addition of polystyrene and copolymers containing polystyrene including rubber-modified high impact polystyrene.
More recently, Haaf et al., U.S. Pat. No. 4,536,500 disclosed adding a styrene resin and an impact modifying agent to polyphenylene ether to improve mold flow properties. The impact modifying agent comprises an elastomeric block copolymer of a vinyl aromatic compound and a conjugated diene and a hydrogenated derivative of the same. These efforts have also improved the foamability of the polyphenylene ether composition. However, it is also known that the addition of polystyrene tends to degrade the inherent flame retardant properties of the polyphenylene ether, and this necessitates the introduction into the composition of still another component, namely, a flame retardant agent.
For example, it has been disclosed in the art that certain halogenated compounds, and brominated compounds in particular, are effective as flame retardant additives for polyphenylene ether-polystyrene blends. An example of such a disclosure is Reinhard, U.S. Pat. No. 3,809,729.
There has also be described in Gallucci and Wroczynski Ser. No. 901,434 filed Aug. 28, 1986, that precompounding of at least one alpha, beta-unsaturated acid ester or maleimide or derivative thereof improves the processability and oxidative stability of polyphenylene ether resin compositions.
Foaming agents are customarily employed to be make moldable compositions including compositions containing polyphenylene ether resins. Foaming agents tend to reduce the weight of a thermoplastic composition to achieve lightweight properties by decomposing under reaction conditions with the evolution of gas. Foaming agents have been used to obtain foamable compositions of a variety of thermoplastic materials as described in, for example, Muller et al., U.S. Pat. No. 3,781,233 and Fox, U.S. Pat. No. 4,280,005 and the U.S. and British patent references cited therein. Lundberg, U.S. Pat. No. 3,947,387 and Liu, U.S. Pat. No. 4,588,754 disclose foamed polymeric products using citric acid, alone or citric acid mixed with sodium bicarbonate as foaming agents.
Despite these efforts, however, there is still an important need in the industry to improve the foamability of polyphenylene ether resins without incorporating other components such as polystyrene which may detract from the properties of polyphenylene ether.
Applicant has now discovered that a high quality foamable composition of polyphenylene ether resin can be obtained by the precompounding thereof with at least one carboxylic acid ester or maleimide or derivative thereof and then adding a suitable foaming agent to the modified polyphenylene ether resin. The present foamable composition retains its flame retardant properties thereby eliminating the necessity of introducing polystyrene and a flame retardant agent into the composition.
Moreover, the compositions can be easily foam-molded to very low densities, e.g., about 40-50% weight reduction compared to standard injection molded parts, using foam molding conditions and blowing agent concentrations which are conventional for other engineering thermoplastic foams.
It is therefore an object of the present invention to provide a polyphenylene ether resin composition with improved foam molding properties.
It is a further object of the invention to improve the foam molding properties of polyphenylene ether resins and to provide products having excellent flame retardancy.
It is a still further object of the invention to provide a polyphenylene ether composition which can be easily foam molded, even in the absence of polystyrene.